Honing

Once the wax has solidified, the wax block is removed from the mold, the identification number is noted and the excess wax is cut off from the block to expose the tissue surface in preparation for actual cutting. This procedure is known as trimming. Only thin slices are taken out at a time to prevent the block from cracking.

The sides, top and bottom of the tissue are trimmed until perfectly level and all sides are parallel, almost to the edge of the tissue. The surface block is then trimmed away until the entire tissue surface has been partly exposed. A heated spatula is held between and the block holder until the wax begins to melt. The spatula is then withdrawn and the block is gently pressed into position. The block is allowed to harden for cutting proper by facing them down in ice cold water or refrigerator for 5 – 10 minutes. The block is then placed in the microtome for cutting.

Trimmed celloidin blocks are mounted on a wooden block holder whose surface has been made rough and serrated with a saw, to promote adhesion. The block holder is coated with 2 – 4% (thin) celloidin and the trimmed block is pressed firmly into position for at least one hour by means of a lead weight or a thread tied around the holder and the block. The seal is hardened and set firm in a chloroform dessicator. The block is re–immersed in 70% alcohol for 30 minutes, trimmed further with a sharp razor or knife, and finally mounted on a microtome for sectioning.

Unlike paraffin embedded tissues, celloidin blocks do not require chilling or refrigeration.

Microtome knives

Trimming and section–cutting are done with a microtome knife, which is available in three basic types or shapes.

1.   Plane–concave (usually 250 mm in length) – one side of the knife is flat while the other is concave. Less concave sides are recommended for cutting celloidin – embedded tissue blocks on a sliding microtome. More concave slides are used to cut paraffin sections on base – sledge, rotary or rocking microtome.

2.   Biconcave knife (usually 120 mm in length) – with both sides concave, recommended for cutting paraffin embedded section on a rotary microtome.

3.   Plane–wedge knife (usually 100 mm in length) – have both sides straight, recommended for frozen sections or for cutting extremely hard and tough specimens embedded in paraffin blocks, using a base–sledge type of sliding microtome.

There is a cutting facet (bevel) found on the tapered edge of all knives, the sides of which are more actually inclined towards each other, than the side proper, forming the actual cutting edge of all knives. The angle formed between the cutting edges is known as the “bevel angle,” normally about 27” to 32”. Such angle is maintained for each knife by means of a slide–on back, a spring–loaded semi–circular metal sheet slipped on the knife with one or more plane surfaces (plane–wedge or plane–concave) to hold the cutting edge at a constant, correct angle during the process of honing and stropping. Each knife should have its own corresponding back which should not be interchanged with another, to keep the bevel angle constant all the time.

Theoretically, the perfect and optimum cutting angle is obtained when two smooth plane surfaces meet at an angle of about 14^o, causing maximum penetration of the tissues. A good cutting edge should be made of good quality steel.  Too soft cutting edges are likely to become dull easily, while too hard edges are likely to produce nicks or jagged edges and irregularities on the knife edge, thereby producing tears or striations on the tissue sections during cutting. A good cutting edge must be able to cut good sections from a paraffin wax block about 2 – 3 micra thick, without any serration noted on examination.

Safety razor blades may be used for partially calcified materials, paraffin and frozen sections. They are readily replaced when dull, and produce similarly good tissue section as those cut with microtome knives. They are, however, unsatisfactory for sections less than 10u.

HONING AND STROPPING                                                                                          

Badly nicked knives with blunted ends have to undergo sharpening in order to ensure optimum sectioning of tissue blocks and prevent gross irregularities on the tissue sections. Jagged edges, if not corrected, will produce tears or striae in tissue sections.

Sharpening of the knife involves two stages, namely:

1.     Honing – involves the removal of gross nicks on the knife edge (coarse honing) to remove blemishes and grinding the cutting edge of the knife on a stone (honing proper) to acquire an even edge.

2.     Stropping – involves the removal of the “burr” or irregularities that have been formed during honing, and the final polishing of the knife edge.

Honing (hard sharpening)

This procedure makes use of a bone, a natural sharpening stone or hard grinding surface (carborundum), which serves to remove nicks and irregularities on the knife edges. Several types of hone may be used:

1.   Belgium yellow – for manual sharpening when cutting edge has been rendered blunt or nicked. This type usually gives the best result.

2.   Arkansas – give more polishing effect than the Belgian yellow.

3.   Fine carborundum – is much coarser than the first two types and is used only for badly nicked knives followed by either one of the first two sharpeners.

The surface of the bone is wiped clean with a soft cloth moistened with xylene in order to remove scattered small particles of stones and metal. It is then covered with a thin film of mineral and clove oil, xylene, liquid paraffin or soapy water for lubrication. The knife is fitted to its corresponding back, placed on one end of the hone, and with the cutting knife edge first, the “heel” (handle end) is drawn obliquely or diagonally towards the operator on the stone until the “toe” (head portion) is reached. The knife is then turned over, and the other surface is again drawn forward, edge first, with heel to toe direction.

Such sequence forms a double stroke, with a knife held obliquely, taking the same precaution to hone the entire length of the knife. Honing is then continued until all the teeth in the knife edge have been eradicated.

In the case of the Minot or Plane–wedged knife, the knife is turned over so as to sharpen the other surface every 10 – 20 strokes. For plane–concave knives, only the concave surfaces should be rubbed on the hone.

Plane–wedge and plane–concave knives are usually provided with backs, to maintain the correct bevel angle throughout honing. Detachable handles may be attached to the knife during sharpening.

A flat circular glass plate with finely powdered aluminum oxide made into paste with water (used as an abrasive) may be used for grinding and removing nicks. Diamantine may also be used for final polishing. The plate glass is usually ¼ to 3/8 inch thick, about 14 inches long and 1 – 2 inches wider than the length of the knife blade to be sharpened. Due to the plate’s relatively greater width, the knife blade does not have to be held obliquely but is pushed and pulled forward and backward at right angles to the transverse diameter of the plate.

Mechanical honing with machines may make use of a vibrating frosted glass plate or a wheel driven by an electrical motor. The knife is pressed against the flat slide of a rotating glass wheel which is being driven by a mechanical device. Approximately 30 double strokes are given each side of the knife to which every gentle pressure is applied. The use of knife sharpening machines although quite expensive, is usually time –saving and produce well–sharpened knives with the uniform bevels.

Precautions observed during honing:

The hone should be long enough (about 8” x 3”) to allow the whole length of the knife edge to be sharpened in a single stroke and wide enough to sufficiently support and prevent the rocking of the knife.

The hone should be lubricated with warm soapy water or fine oil before using. It is then washed, preferably with water to remove all metal particles that may have been collected during the process. The washing fluid used must flow rapidly enough so that the metal chips are removed between strokes and a clean hone is presented every time.

The pressure on the knife should be gentle and steady to keep it from rocking. The number of strokes usually amounts to 20 – 30 times in each direction, depending upon the condition of the knife. Badly nicked knives require greater and longer honing than less irregular knives.

The hone should be cleaned before, during and after use. A black film that develops in the hone usually is imparted by the knife that is being sharpened, and should be brushed out with a good nailbrush in running water, which may either be plain or soapy, until the hone is thoroughly cleaned. After its use, the hone must be washed with warm soapy water, dried, and kept in a box, to protect it from dust while it is not in use.

After honing, wipe off the oil or soap from the knife with xylene. Then strop it thoroughly.

Stropping

Stropping is the process whereby the “burr” formed during honing is removed and the cutting edge of the knife polished.

The purpose of stropping is to polish and sharpen the cutting edge; while that of honing is to remove the irregularities from the knife. If the knife has become dull and blunt, but is free from nicks or teeth, it is usually only necessary to strop it. For delicate work, the knife is stropped before every object is sectioned.

A paddle strop made up of the best quality horseleather, firmly attached to a solid back, in order to prevent sagging is preferred.

The procedure is the reverse of honing. The knife is first fitted with its appropriate back, then laid obliquely on the strop and with the cutting edge behind, (EDGE LAST) is pushed backward and drawn forward in a toe to heel direction. Around 40 – 120 double strokes are usually required.

In the case of plane–wedge or Minot knives, the knife is turned around at the end of each stroke, so as to sharpen each surface alternately. For plane – concave knives, only the concave surface should be stropped.

Precaution observed in stropping

The knife should always be wiped clean with a soft cloth before and after a series of stropping strokes and before changing from a coarse to a fine strop, to remove particles which may have been taken off the knife. After stropping is satisfactorily completed, the knife is kept covered in a suspension box to prevent the settling of dust and grit on its surface, causing damage to the knife edge. The knife should not be allowed to rest on its sides since this may also damage the cutting edge.

Pressure during the first stropping strokes should be quite light, since the natural compressibility of the leather is what actually does the work. Only a gentle pressure should be applied while the knife is held steady on the strop and damage the cutting edge.

Speed in stropping should be avoided. One full second should be allowed for each stroke to avoid injury to the strop and the knife.

Leather strops are usually dry and require oiling before they are used. Strops are usually treated with vegetable oil (e.g. Castor oil) applied into the back of the strop. NOT the surface.  The strop should not be used for at least 24 – 48 hours after treatment. Too much oil will make the stropping surface slippery and will render the procedure unsatisfactory. To remove excessive oil from the strop, its surface is scrapped with a blunt instrument, e.g. back of the knife.

Mineral oil is not recommended and should never come in contact with a strop since it will tend to blister and destroy the leather. One drop of mineral oil will spoil the polish of that area, and produce a permanent blemish on the strop.

Stropping surfaces should be firm and not loose, to prevent the turning of the knife’s edge. Hence, strops are usually mounted on wooden canvass and covered with a flat pad to prevent them from slugging.

Wax must not be allowed to come in contact with the strop. With an applicator, the used knife blade should be washed and flushed with xylene. The knife is then dried off, by wiping the knife on soft paper or cloth, (never wipe the paper or cloth on the knife). The procedure is again repeated with fresh xylene and a fresh sheet of paper or cloth, until the wax has been removed.